Lithostratigraphic and allostratigraphic framework of the Cambrian–Ordovician Potsdam Group and correlations across Early Paleozoic southern Laurentia

Abstract

The Potsdam Group is a Cambrian to Lower Ordovician siliciclastic unit that crops out along the southeastern margins of the Ottawa graben. From its base upward, the Potsdam consists of the Ausable, Hannawa Falls, and Keeseville formations. In addition, the Potsdam is subdivided into three allounits: allounit 1 comprises the Ausable and Hannawa Falls, and allounits 2 and 3, respectively, the lower and upper parts of the Keeseville. Allounit 1 records Early to Middle Cambrian syn-rift arkosic fluvial sedimentation (Ausable Formation) with interfingering mudstone, arkose, and dolostone of the marine Altona Member recording transgression of the easternmost part of the Ottawa graben. Rift sedimentation was followed by a Middle Cambrian climate change resulting in local quartzose aeolian sedimentation (Hannawa Falls Formation). Allounit 1 sedimentation termination coincided with latest(?) Middle Cambrian tectonic reactivation of parts of the Ottawa graben. Allounit 2 (lower Keeseville) records mainly Upper Cambrian quartzose fluvial sedimentation, with transgression of the northern Ottawa graben resulting in deposition of mixed carbonate–siliciclastic strata of the marine Rivière Aux Outardes Member. Sedimentation was then terminated by an earliest Ordovician regression and unconformity development. Allounit 3 (upper Keeseville) records diachronous transgression across the Ottawa graben that by the Arenigian culminated in mixed carbonate–siliciclastic, shallow marine sedimentation (Theresa Formation). The contact separating the Potsdam Group and Theresa Formation is conformable, except locally in parts of the northern Ottawa graben where the presence of localized islands and (or) coastal salients resulted in subaerial exposure and erosion of the uppermost Potsdam strata, and accordingly unconformity development.

Early Paleozoic rifting and reactivation of a passive-margin rift: Insights from detrital zircon provenance signatures of the Potsdam Group, Ottawa graben

“…We thank Landing et al for their critique of our work and, based on their comments, address what we believe are the principal points of disagreement. Although their comments appear to be directed at this paper (Lowe et al, 2018), which outlines the provenance and lithotectonic record of the Potsdam Group, their comments are principally aimed at an earlier foundational paper published in the Canadian Journal of Earth Sciences (Lowe et al, 2017) that outlines the stratigraphy and facies of the Potsdam Group. The comments of Landing et al range from minor nomenclatural discrepancies to objections to our stratigraphic framework and facies models, which we address next.…”

“…The first major attempt to provide a genetic basin-wide framework and unified stratigraphic nomenclature was that of Sanford (2007); later documented in Sanford and Arnott, (2010). The objective of subsequent work by Lowe (2016); published in Lowe and Arnott, (2016), Lowe et al (2017), and Lowe et al (2018), was to test that framework, make modifications where appropriate, and elucidate the stratal evolution of the Potsdam Group in the Ottawa graben. These basin-specific works outline, for the first time, the uniqueness of the stratigraphy of the Potsdam Group in the Ottawa graben that were previously unaccounted for and documents the profound tectonic overprint on the eustatic and climatic variations that controlled sedimentation here and on the coeval Laurentian shelf and slope.…”

“…The first issue of Landing et al, that we have redefined earlier units "without comment," thus creating confusion, is inaccurate. Redefinitions of the Potsdam Group and its constituent units and discussion of how and why these replace earlier nomenclature are discussed and justified throughout Lowe et al (2017). Secondly, the framework of Lowe et al (2017) was not intended to generate confusion, but rather limit it by resolving some issues with earlier correlations stemming from local jurisdictional treatment of the succession and the incorrect correlation of units with different detrital composition, stratigraphic position, and depositional environments.…”

“…Redefinitions of the Potsdam Group and its constituent units and discussion of how and why these replace earlier nomenclature are discussed and justified throughout Lowe et al (2017). Secondly, the framework of Lowe et al (2017) was not intended to generate confusion, but rather limit it by resolving some issues with earlier correlations stemming from local jurisdictional treatment of the succession and the incorrect correlation of units with different detrital composition, stratigraphic position, and depositional environments. The principal purpose of redefining the stratigraphy of the Potsdam Group was to provide a single, regionally consistent stratigraphic framework that would allow for a more comprehensive interpretation and deconvolution of the eustatic, climatic, and tectonic controls on sedimentation (in accordance with Article 7(e) of the North American Stratigraphic Code, "For geologic units that cross local and international boundaries, a single name for each is preferable to several"; NACSN, 2005).…”

“…The lithology, facies, and detrital composition of these strata are consistent with the Ausable elsewhere. Landing et al (2009) also describe the lowermost exposed strata on the Old Military Turnpike as "pink to whitish feldspathic quartz arenite" with "granule and small pebble quartz and feldspar grains" and "trough cross-bedding"; it is unclear how this differs from the Ausable as defined by Lowe et al (2017). Landing et al (2009) also note ~3 m of basal "sandstone" in their log of hydrological wellbore 1-02, although the detrital composition is not reported.…”